NASA’s Kepler spacecraft, which is looking for planets outside our solar system, has found three of the smallest exoplanets yet. They’re all smaller than Earth — their radii are 0.78, 0.73 and 0.57 times that of Earth’s — and the smallest is about the size of Mars.

They’re very close to their star, too close to be good candidates for life because liquid water can’t exist on them, and their star is a red dwarf. But what makes them special is that they are so small.

The techniques we use for finding exoplanets work best with large, massive planets, as I’ve mentioned before. It’s only been recently that we’ve been able to find planets of around Earth’s size, and especially those that are likely to be rocky, terrestrial planets like Earth. The three that Kepler’s found fall into that small-rocky territory. That’s crucial — we’ve mainly found gas giants, which made astronomers wonder if our solar system was an unusual one because it has so many rocky planets in it. As we find more rocky planets, we learn more about how solar systems form and help us understand if life here on Earth is a fluke or likely to be repeated across the galaxy.

Scientific America has a good discussion of why Kepler was able to find these three planets, if you’d like to know more. What’s really exciting is that Kepler may be able to discover planets as small as our Moon, and given the firehose of data coming out of Kepler, there are likely many more discoveries where these three came from.

2 Comments

OK, Stephen, I need some info. So I’ve seen these articles before, and they (likewise) mentioned that there wouldn’t be liquid water on them.

But doesn’t that assume that they’re rotating? If these planets are that close to their star, isn’t it possible that they’re tidally locked? The star-facing side would be crazy-hot. The void-facing side would be frozen.

But wouldn’t there be a particular set of longitudes where temperatures were right for liquid water along the (permanent) day/night transition area?

I’m sure there are other issues. Being that close to the star, I’m sure radiation is high. I don’t know what kind of solar wind a red dwarf puts off, but that might have stripped any atmosphere. You could theorize some kind of stupid-large magnetic field, though, that might still protect from some of those effects.

So I could see the right temperature, right gravity, right atmospheric pressure to allow liquid water in a “twilight” area.

They could be tidally locked, though that would take a while to occur. I don’t know off-hand the age of these planets or star system, so I don’t know how likely it is for any of these to have reached tidal lock. If they are tidally locked, then yeah, you might have liquid water on the dividing line between day and night. But if you have an atmosphere you’d have terrific wind forces being driven by the heat being pumped on one side and the loss of heat on the other.

I think you can come up with a scenario where there’s the possibility of liquid water, but in general it’s highly unlikely. That makes it easier just to say, “They don’t have liquid water,” with the caveat of “we don’t know for sure 100%, but it’s extremely likely”.